A projected optical image is always degraded by the projection system due to optical aberrations and to the finite wavelength of light. Aberrations can be reduced by design, but the influence of diffraction of the light due to its finite wavelength puts a limit to the resolution and fidelity that can be achieved. This is well-know and many optical devices operate at the diffraction limit, e.g., microscopes, astronomical telescopes, and various devices used for microlithography. In microlithography, the size of the features printed limit the density of features that can added to the workpiece and therefore the value that can be added to the workpiece at each step. Because of the strong economic forces towards smaller and more numerous features on the workpiece, the optics used in lithographic processes are extremely well designed and limited only be the underlying physics, i.e., diffraction.
Many projection systems are designed as incoherent projectors. Coherence in this application means spatial coherence and is a way of describing the angular subtense of the illumination of the object (the mask, SLM, etc.) in relation to the angular subtense picked up by the projection lens. Incoherent in this sense means that the illumination as seen from the object has a larger angle range than what is transmitted by the projection lens. Tuning of the illumination angles has a profound influence on the image. The incoherent projection gives an image that is pleasing to the eye with a gradual fall-off of the contrast as one gets closer to the resolution limit. But for technical purposes, this fall-off means size errors for everything close to the resolution limit and the smallest features that can be printed with good fidelity are far larger than the resolution limit. In photography, the optical resolution is often determined as the smallest high-contrast object features that appear with any visible contrast in the image. For microlithography, the resolution is pragmatically determined as the smallest features that print with enough quality to be used. Since microlithographic patterns are imaged onto a high-contrast resist and the resist is further raised by the etching process, the quality in the image is almost entirely related to the placement and quality of the feature edges. Resolution is then the smallest size that, given the constraints of the process, gives acceptably small size errors (“critical dimension errors” or “CD”) and acceptably large process latitude. Resolution is, therefore, in lithography a stricter definition than in photographic imaging and is more determined by residual CD errors than by the actual limit of the optical system.
With partially coherent illumination, FIGS. 1 a-b, the angular range of the illuminator is limited to smaller than is accepted by the projection lens. This raises the useful resolution by introducing some amount of coherent “ringing” at the edges of the image. These ringing effects also affect neighboring edges and the image shows so called proximity effects: the placement of every edge depends on the features in the proximity to it. The illumination angles, i.e., the distribution of light in the illuminator aperture, can be tuned for higher useful resolution at the expense of more proximity effects and it becomes a trade-off between resolution and image fidelity.
The lithographic industry has raised the resolution by tuning the illumination and correcting residual errors by as much optical proximity processing in the mask data as it takes. As the requirements for both resolution and fidelity have risen, the OPC processing has become very extensive with model-based simulation of essentially whole chips. The OPC processing can be done using specialized software running on computer farms and still take several hours or even days. With OPC adjustments, a more aggressive illuminator can be used. Some historic figures illustrate this.
In the early 1990s, printed linewidths in microlithography were typically 0.70*lambda/NA, where lambda is as normal the wavelength of the light and NA is the sine of the opening half-angle of the projection lens. The factor lambda/NA is a constant for a particular type of equipment. In 2004, industry is printing 0.40*lambda/NA with OPC, sometimes down to about 0.30*lambda/NA, which means that five times more features can be printed using exactly the same optical limitations (lambda and NA). This requires heavy OPC correction in the masks. Correcting for the effects of the printing on the wafer adds cost, overhead and lead time. The extensive OPC corrections currently used in state-of-the-art products have produced an explosion of the data file size. At the 90 and 65 nm design nodes, pattern data files may be 50 Gbyte or more in size and even the transmission and storage of the files becomes a burden to the design houses and mask shops. Adding one more layer of OPC corrections for the printing of the mask in an SLM-based pattern generator would add more cost, overhead and make the lead time even longer.
Therefore, there is a need in the art for an improved method for printing highly accurate patterns. One use of the disclosed technology is to optimize the optics in order to lessen or even remove the need for optical proximity correction. It can be applied in the maskwriter, in a direct-writer or in mask-based lithography.